Add GN output prefix override and allow empty output extensions.

tools/gn/target.cc

Index: tools/gn/target.cc
diff --git a/tools/gn/target.cc b/tools/gn/target.cc
index 005dd973b2961cb4f445a1a61bb302a85df03b71..7d6e530b76dc1d2c77a25f296b3c095b2ef2dfd9 100644
--- a/tools/gn/target.cc
+++ b/tools/gn/target.cc
@@ -212,6 +212,8 @@ bool RecursiveCheckAssertNoDeps(const Target* target,
 Target::Target(const Settings* settings, const Label& label)
     : Item(settings, label),
       output_type_(UNKNOWN),
+      output_prefix_override_(false),
+      output_extension_set_(false),
       all_headers_public_(true),
       check_includes_(true),
       complete_static_lib_(false),
@@ -369,8 +371,10 @@ std::string Target::GetComputedOutputName(bool include_prefix) const {
   if (include_prefix) {
     const Tool* tool = toolchain_->GetToolForTargetFinalOutput(this);
     if (tool) {
-      // Only add the prefix if the name doesn't already have it.
-      if (!base::StartsWith(name, tool->output_prefix(),
+      // Only add the prefix if the name doesn't already have it and it's not
+      // being overridden.
+      if (!output_prefix_override_ &&
+          !base::StartsWith(name, tool->output_prefix(),
                             base::CompareCase::SENSITIVE))
         result = tool->output_prefix();
     }